I blogged way back in December about a study by David Tilman at the University of Minnesota which showed that “mixtures of native perennial grasses and other flowering plants provide more usable energy per acre than corn grain ethanol or soybean biodiesel and are far better for the environment” (quoted by Mongbay.com). This was actually the latest in a series of long-term experiments by Tilman looking at the connection between diversity and ecosystem function in grasslands (see also a recent paper by Hector et al. in Functional Ecology). This connection has been less investigated in agricultural systems, however. Which is the reason behind a recent paper in the Journal of Applied Ecology from researchers at Michigan State University ((R.G. Smith, K.L. Gross. 2007. Assembly of weed communities along a crop diversity gradient. Journal of Applied Ecology 44 (5), 1046–1056.)).
The researchers grew maize, soybean and winter wheat both in continuous monoculture and in various annual rotations, with and without cover crops, but always without fertilizers and pesticides. “Increasing cropping system diversity is one strategy for reducing reliance on external chemical inputs in agriculture and may have important implications for agro-ecosystem functions related to the regulation of weed populations and community assembly.” So they then measured weed populations every year, and assessed soil resources at the end of the study. They concluded that
Increasing crop diversity in the absence of external chemical inputs can result in changes in soil resource availability without a concomitant increase in the abundance of weeds or a shift to weed communities that are more difficult to manage.
That sounds a bit like damning with faint praise, frankly. But I’ve only read the abstract so far. I’ll try to get hold of the full paper tomorrow and see if other possible effects of increased crop diversity were measured.
LATER: Not really much to add. The main conclusion seems to be that if you enhance crop diversity — to improve soil fertility or the abundance of natural enemies — you won’t necessarily have any bigger a weed problem on your hands.